Measurements of the production cross-section for a Z boson in association with b -jets in proton-proton collisions at s

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Received: March 27, 2020 Accepted: June 15, 2020 Published: July 7, 2020

The ATLAS collaboration E-mail: [email protected] Abstract: This paper presents a measurement of the production cross-section of a Z √ boson in association with b-jets, in proton-proton collisions at s = 13 TeV with the ATLAS experiment at the Large Hadron Collider using data corresponding to an integrated luminosity of 35.6 fb−1 . Inclusive and differential cross-sections are measured for events containing a Z boson decaying into electrons or muons and produced in association with at least one or at least two b-jets with transverse momentum pT > 20 GeV and rapidity |y| < 2.5. Predictions from several Monte Carlo generators based on leading-order (LO) or next-to-leading-order (NLO) matrix elements interfaced with a parton-shower simulation and testing different flavour schemes for the choice of initial-state partons are compared with measured cross-sections. The 5-flavour number scheme predictions at NLO accuracy agree better with data than 4-flavour number scheme ones. The 4-flavour number scheme predictions underestimate data in events with at least one b-jet. Keywords: Hadron-Hadron scattering (experiments) ArXiv ePrint: 2003.11960

Open Access, Copyright CERN, for the benefit of the ATLAS Collaboration. Article funded by SCOAP3 .

https://doi.org/10.1007/JHEP07(2020)044

JHEP07(2020)044

Measurements of the production cross-section for a Z boson in association with b-jets in proton-proton √ collisions at s = 13 TeV with the ATLAS detector

Contents 1

2 The ATLAS detector

3

3 Data set and simulated event samples 3.1 Data set description 3.2 Simulated event samples for signal and background processes 3.3 Theoretical predictions

4 4 4 6

4 Event selection 4.1 Correction factors applied to simulation and corresponding uncertainties

9 11

5 Background estimation 5.1 Extraction of the cross-section for Z-boson production in association with light-jets and c-jets

12

6 Kinematic distributions

17

7 Correction to particle level

18

8 Uncertainties in the cross-section measurements

21

9 Results 9.1 Inclusive cross-sections 9.2 Differential cross-sections for Z + ≥ 1 b-jet 9.3 Differential cross-sections for Z + ≥ 2 b-jets

23 24 24 29

10 Conclusion

33

The ATLAS collaboration

41

1

14

Introduction

The measurement of the production rate of a Z boson in association with jets originating from b-quarks1 (Z + b-jets) in proton-proton (pp) collisions provides an important test of perturbative quantum chromodynamics (pQCD). Current predictions for Z +b-jets production are known at next-to-leading-order (NLO) accuracy in pQCD, and they can be derived in either a 4-flavour number scheme (4FNS) or a 5-flavour number scheme (5FNS) [1–4]. 1

Unless otherwise mentioned, it is implicitly assumed that b-quark refers to both b-quark and ¯b-antiquark.

–1–

JHEP07(2020)044

1 Introduction

2

A c-jet is a jet originating from a c-quark. ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point